Differentiation between intracerebral hemorrhage and edema will be one of the useful contributions of this study. Macacus rhesus monkeys from which the calvarium was removed and the scalp allowed to heal over the intact dura mater were used as an experimental model. Autogenous blood was injected into the temporal region through a small gauge needle. Ultrasound scanning was carried out after each increment of blood was injected. Injections of blood in the range of 0.1 to 0.3 ml. could be detected. Langfit's method of freezing the cerebral cortex to produce hemorrhage with a progressively enlarging area of edema around it was visualized with ultrasound serially for 30 days after lesioning. Hemorrhage can be readily identified as it produces strong echoes. Edematous areas seem to produce fewer than normal echoes, and can usually be identified with shifts which are visible in the falx and tentorium. It seems likely that localization and extent of edema can be identified, and the effectiveness of various anti-edema therapies assessed. Prior knowledge of the nature and approximate location of lesions in these brains has made detection and differentiation possible. Hemorrhage or edema in brains where the trauma site is not known will make it more difficult to identify small lesions, and more experience will be needed. Hemorrhage of 1 ml. or more in size can be identified in a monkey brain, but lesions of 0.1 ml. size cannot be differentiated from echoes produced by normal structures without some prior knowledge of the location. The lower limits of resolution and identification have not been determined but will probably vary with the location of the lesion within the brain. In the experiments thus far performed, there was prior knowledge concerning the nature and location of the lesion, which materially aided in the detection and differentiation.